The present invention relates to an exhaust gas processing device for a fuel cell, and more particularly to an exhaust gas processing device for a fuel cell, which processes hydrogen purged from a fuel cell system where hydrogen is utilized as a fuel for a power source of an electric vehicle.
In general, a fuel cell defines the cathode and the anode at both ends of a proton exchange membrane or polymer electrolyte membrane (PEM membrane) so that oxygen in the air is supplied to the cathode while hydrogen contained in a fuel gas is supplied to the anode to thereby generate electricity by the electrochemical reaction therebetween.
In a fuel cell system using such a fuel cell as a power source for an electric vehicle, and in cases, for example, where pure hydrogen (herein after simply referred to “hydrogen”) is used as fuel, a recirculation system is adopted for the purpose of improving utilization efficiency of hydrogen to be supplied to the fuel cell and thereby improving fuel economy. For example, Japanese Laid-open Patent Application No.6-275300 (see Page 4 and FIG. 1) discloses such a recirculation system. To recirculate hydrogen, the recirculation system may be provided with an ejector which generates negative pressure for sucking hydrogen or a vacuum pump.
During long-term recirculation of hydrogen, the concentration of impurities such as nitrogen increases in the recirculation system, leading to deterioration in generation efficiency. Also, moisture retained in the fuel cell system makes a flow of hydrogen clogged in the anode pipe system. For this reason, purging is required to discharge impurities such as nitrogen and water to the atmosphere. However, because hydrogen is filled in the anode pipe system, high concentration hydrogen is also discharged to the external during the purging. In order to prevent hydrogen (purged hydrogen) from being discharged to the atmosphere, purged hydrogen is introduced into an exhaust fuel diluter and mixed with cathode exhaust gas. Therefore, purged hydrogen is diluted to a lower concentration and then discharged to the atmosphere.
Japanese Laid-open Patent Application No.11-191422 (column [0024] and FIG. 2) discloses a technique wherein hydrogen discharged from the fuel cell is mixed with air in an exhaust fuel diluter and diluted hydrogen is discharged to the atmosphere.
However, in a case where purged hydrogen that is discharged to the atmosphere is intermittently fed to the exhaust fuel diluter, because the concentration fluctuation of purged hydrogen in the exhaust fuel diluter becomes greater with elapse of the time, the concentration fluctuation of purged hydrogen that is mixed with cathode exhaust gas and diluted prior to being discharged to the atmosphere also becomes greater.
In this regard, even in a case where purged hydrogen that is discharged to the atmosphere is introduced intermittently into the exhaust fuel diluter, it is desired that the concentration fluctuation of the purged hydrogen does not become greater with elapse of the time.
It is also desired that the hydrogen concentration in emission gas is highly accurately controlled to exert the inherent performance of the exhaust fuel diluter in order to keep the hydrogen concentration in the emission gas that is discharged from the exhaust fuel diluter to the atmosphere not greater than a predetermined limit value and to dilute and process as much purged hydrogen as possible.